Preheat oven at 250°F.
Spray a large mixing bowl lightly with no-stick cooking spray and place popped popcorn inside; set aside.
Combine butter, brown sugar, corn syrup, and salt in a heavy 2-quart saucepan.
Bring to a boil over medium heat, stirring constantly.
Lower heat and boil 5 minutes, without stirring.
Remove from heat and stir in baking soda.
Gradually pour over reserved popped corn, mixing well to coat.
Spread popcorn evenly into a rimmed 17- x 12-inch baking pan.
Bake for 1 hour, stirring every 15 minutes.
Remove from oven and cool completely.
Store in airtight container.
